Permanent magnet galvanometer



Aug; 16, 1938. M. w. SCHELDORF 2,127,427

PERMANENT MAGNET GALVANOMETER Filed July 51, 1937 Inventor: Marvel W. Scheldorfi Patented Aug. l6, 1-938 2.12am PERMANENT MAGNET oarvanonm'raa Marvel w. Scheldori', Schenectady, N. ll., assignor to General Electric fllompany, a corporation I or New York Application July or, 1937, Serial No. 156,732 f 4 Claims. (Cl. I'll-95) My invention relates to permanent-magnet galvanouieters o! the balanced armature type. such as those employed in radio loud speakers, magnetic pick-up devices, electrocardiographs,

% and the hire, is .1. its object is to provide a galvanonleter whose response is more nearly proportional to the variation of the current in its euciting winding than has been the case with prior galvanometers of this type.

W The ieatures oi the invention which are believed to he novel and patentable will be pointed out in the claims appended hereto. For a better understanding otmy invention, reference is made in the ioilowing description to the accompanying it drawing wherein a side view ot a preferred emhooliinent oi my invention is illustrated.

lihe galvanoineter illustrated is oi. the balanced armature type. It includes a pair oi U- shaped colt-iron pole-pieceparts it and it with their open sides facing each other but separated to provitle a relatively large armature air gap.

in which a magnetic vane armature it is symmetrically mounted. The armature is biased to a central neutral position by suitable restraining means which may comprise a torsional rod it on which the armature is fixed at its center. The rotl iii may be clamped at one or both of its ends, one" such clamp being indicated bash oi the armature at it. an exciting winding 3 made in two coils it and it surrounds opp lsite enol rtions oi the armature it. These coils will .be connected'iu series and excited by a current, such as the voice current of a radio loud spealter, tending to poiarize the vane armature it in the direction oi its length. Also a mirror it moved by the armature and a light ray indicating system such as a lamp ti and scale it ay he used.

A horseshoe-shaped permanent magnet ii is m provided to create a unidirectional magnetometive torce between the pole pieces of the two parts it and it as indicated by the designations it and S. it now the excitation of coils ill and ii is such as to polarize the armature to produce a south pole at its right end and north pole at its left end as indicated; the armature willtend to twist counterclockwise and, ii

the exciting current is reversed, it wlll tend to turn in the opposite direction against the restraining iorce of the torsional shaft l3.

in the prior art device of this type, the permanent magnet ll abutted directly against the softiron pole-piece parts it and it so that the only air gap of any consequence in the permanent 55 magnet circuit was the gap between the pole pieces it and it. Such prior art arrangement,

while operative, was not very satisfactory for some purposes because the armature was quite unstable except, of course, in a central position with no exciting current flowing in coils liand it. The armature had a strong tendency to snap over against and bridge the pole pieces, the position indicated in dotted lines, as soon as there was any" current flow in the exciting winding. This tendency was due largely tothe tendency of the armature to move to close the only air gap in the permanent magnet flux circuit, and

this tendency increased as the armature de;

parted from a'cent'ral position, making it impracticable to offsetthis' magnetic pull by any simple restraining device. For, these reasons, the useful working range of the armature was quite limited and the deflecting force dimcult to match and control by a restraining spring.

I have found that, if the permanent magnet circuit is provided with additional air gaps, as, for example, by placing nonmagnetic spacers it and it between the permanent magnet ll arid. the pole-piece parts it and it, greatly improved results are obtained. The permanent magnet must be of a larger size or produce a greater total flux than before because now appreciable flux leaks across between the pole pieces of the permaneutmagnet asindicated by the dotted lines til. However, the improved results more than ofiset the necessity of using a. stronger permanent magnet.

It is evident that, with no" reluctance gap or its equivalent at it and it when there is a deflection oi the armature from neutral position maximum reluctance, the permanent magnet flux increases and the magnetic pull on the armature increases not only because of the reductionin armature air gap reluctance wlth'deflectlon but also because of the increase in permanent-magnet. flux with such decrease in reluctance; that "before and the increase in permanent magnet flux is correspondingly less. The change in reluctance of the magnetic circuit due to rotation of the armature from maximum reluctance to a lower reluctance position is thus minimized. The

instability of the armature is materially dereased and its sensitivity to current variations and its magnetic. regulation characteristics greatly improved. It is possible to secure much better results by using a large magnet at H with the air gap at It and i9 than with a magnet weak enough to place directly on the pole-piece parts,

it and it without encountering instability.

The relative dimensions of the parts and of -the air gaps shown in the drawing are satisfactory, but the invention; is by no means limited to the relative dimensions illustrated. The total a mature I! in the neutral position.

such as H and I! to the length of air gap in series at 18 and I 9 should preferably be not greater and may be less than the length of air gap between the north and south pole pieces of the galvanometer with ar- The total length of air gap in the useful flux path should obviously not be greater and should preferably be less than the leakage flux gap at 20. With the relative dimensions shown in the drawing, there is a negligible change in flux density across the armature air gap when the armature l2 moves from the neutral to an extreme position,

and this useful flux is comparable in magnitude to'the leakage of flux of the permanent magnet II, the greater portion of which traverses the path indicated at 20. If I make gaps I8 much larger in proportion to the other gaps than here represented, beneficial results as regards stability will still be obtained but this will call for a further increase in the size, strength and cost of the permanent magnet that ceases to be justifiable. The particular air gap dimensions and strength of flux to be used in any particular case will depend upon the degree of stability and sensitivity desired and the use for which the device is intended.

'The strength of the permanent magnet used may be such that the flux density in the useful air gap is increased in comparison to prior galvanometers of this general type. In such prior devices it was the practice to fit the pole pieces permanent magnet as perfectly as possible. The result was that any increase in flux strength for the purpose of increasing sensitivity also decreased the stability. I intentionally provide air gaps at points i8 and I9 and increase the strength of the permanent magnet used, but I do not wish to limit my invention to any particular gap dimensions and strength of permanent magnet because any appreciable gap at the points l8 and i9 improves the relation between sensitivity and stability and is highly beneficial up to the point where this expedient becomes impracticable due to an unreasonable increase in size and cost of the permanent magnet.

In accordance with the usual practice, the magnet i1 is made of high grade permanent magnet material and is permanently magnetized to provide an efiicient permanent magnet. The pole pieces l0 and II are made of soft iron and the armature I2 is preferably made of the best grade of soft, high permeability magnetic material obtainable.

By means of my invention, the useful range of vibration of the armature is increased, its defiection is more nearly proportional to the excitation current, and a more suitable restraining spring may be used.

It is evident that the structural relation of the parts may be materially changed without departing from the principle of the invention.

In accordance with the provisions of the patent statutes, I have described the'principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof but I desire to have it understood that the apparatus shown is only illustrative and that the invention may be carried out by other means.

What I claim as new and desire to secure. by Letters Patent of the United States is:

1. A galvanometer comprising, a magnetic cirsaid gap,

cuit excited by a permanent magnet and having pole pieces defining an air gap, a magnetic vane armature movably supported in said gap, means for resiliently biasing said armature to a neutral position of maximum reluctance in the flux path across said air gap, and an exciting winding which, when energized, polarizes said armature causing it to turn against its bias to a lower reluctance position, said magnetic circuit containing an additional appreciable and intentional air gap for the purpose of minimizing the permanent magnet flux change in said magnetic circuit whichis due to the movement of said armature to a lower reluctance position.

2. A galvanometer having a magnetic circuit containing an armature air gap and a permanent magnet for producing a flux across said air gap, a magnetic vane armature pivoted in said air gap, said vane being resiliently biased to a neutral maximum reluctance position with respect to the permanent magnetic flux crossing and an exciting coil which, when energized, polarizes said vane causing itto turn from a neutral maximum reluctance position against its bias to an unstable lower reluctance position with respect to the permanent magnet flux crossing said air gap, said magnetic circuit including additional appreciable and intentional air gaps to minimize the change in reluctance of the magnetic circuit caused by such movement of said armature.

3. A galvanometer comprising, soft-iron pole pieces separated by an air gap, a permanent magnet for producing a unidirectional flux between said pole pieces across said gap, a magnetic vane-armature pivoted in said air gap, a spring biasing said armature to a neutral position of maximum reluctance with respect to the permanent magnet flux crossing said gap, said armature being movable in opposite directions from such position to lower reluctance positions including positions where it may bridge said pole pieces, an exciting winding which, when energized, polarizes said magnetic vane armature causing it to turn from its neutral position to intercept more of the permanent magnet flux crossing said gap and lowering the reluctance of said magnetic circuit, and nonmagnetic spacers between said permanent magnet and said softiron pole pieces for the purpose of minimizing the change in reluctance of the magnetic circuit due to such movement of said armature.

4. A galvanometer comprising, pole pieces separated to form an armature air gap,

a permanent magnet forming with said pole pieces a magnetic where thevane may bridge said pole pieces, the

permanent magnet being of horseshoe shape and being magnetically spaced from the pole pieces by reluctance gaps permitting a substantial leakage flux between its pole pieces which does not cross the armature air gap, and a coil which, when energized, polarizes the magnetic vane armature and causes it to-deflect from its neutral position. 

